Metal Fabrication

Plasma Cutting

Plasma cutting is a melting process, compared to flame cutting which is a combustion process. A gas jet in the plasma melts and expels the material from the kerf. During the process an electric arc burns between an electrode and the workpiece. The electrode tip is placed in a water or air cooled gas nozzle in the torch. The plasma gas is conducted through the nozzle. The arc and the plasma gas are forced to pass through a very narrow orifice in the tip of the nozzle. The gas is heated and ionised. The concentrated plasma jet which is formed has a temperature of up to 30 000°C together with a high velocity. When the plasma jet hits the workpiece the heat is transferred due to recombination (the gas reverts to its normal state). The material melts and is expelled from the kerf by a flow of gas. One limiting factor for the plasma process has been the high investment costs. The trend today is that more simple and inexpensive machinery is used, making plasma cutting a realistic alternative to other cutting methods. The selection of gas or gases for plasma arc cutting is based on such factors as the required quality of the cut, the thickness of metal to be cut and the gas cost. For cutting thin metal a single gas flow is often used to provide both the plasma and the arc shielding, but for cutting thicker metal, dual gas flows are used. The single gas flow may be air, nitrogen, nitrogen/hydrogen, oxygen or argon. The dual gas flows may be nitrogen, nitrogen/hydrogen, oxygen, argon or argon/hydrogen mixtures.

Using Air Products’ ultra high purity oxygen for plasma cutting of steels can yield a 15% increase in cutting speeds without changing your equipment.

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Plasma cutting is a melting process, compared to flame cutting which is a combustion process. A gas jet in the plasma melts and expels the material from the kerf. To initiate the process and ionise the gas, a pilot arc must be generated. The pilot arc heats the plasma gas and ionises it. Since the electrical resistance of the main arc is lower than that of the pilot arc, the main arc ignites and the pilot arc automatically extinguishes. Air cutting was introduced in the early 1960s for improved cut quality on mild steel. Air was readily available, cheap and worked well as a plasma gas, since it contained a mixture of roughly 80% nitrogen and 20% oxygen. The oxygen in the air provided additional energy through an exothermic reaction with molten steel. This additional energy increased cutting speeds by about 25% over plasma cutting with nitrogen. Although the process could be used to cut stainless steel and aluminium, the cut surface on these materials was heavily oxidised and unacceptable for many applications without secondary operations such as grinding.

Plasma cutting is a melting process, compared to flame cutting which is a combustion process. A gas jet in the plasma melts and expels the material from the kerf. To initiate the process and ionise the gas, a pilot arc must be generated. The pilot arc heats the plasma gas and ionises it. Since the electrical resistance of the main arc is lower than that of the pilot arc, the main arc ignites and the pilot arc automatically extinguishes. Air cutting was introduced in the early 1960s for improved cut quality on mild steel. Air was readily available, cheap and worked well as a plasma gas, since it contained a mixture of roughly 80% nitrogen and 20% oxygen. The oxygen in the air provided additional energy through an exothermic reaction with molten steel. This additional energy increased cutting speeds by about 25% over plasma cutting with nitrogen. Although the process could be used to cut stainless steel and aluminium, the cut surface on these materials was heavily oxidised and unacceptable for many applications without secondary operations such as grinding.

Cutting Gas Mixtures

Other gas combinations can be used in plasma arc cutting processes. Benefits vary depending on the material being cut and the plasma and shield gas being used. Argon-Hydrogen mixtures to obtain better cut quality on thicker stainless steel. Nitrogen/Argon-Hydrogen also give better metallurgical effects on thicker stainless steel.

Other gas combinations can be used in plasma arc cutting processes. Benefits vary depending on the material being cut and the plasma and shield gas being used. Argon-Hydrogen mixtures to obtain better cut quality on thicker stainless steel. Nitrogen/Argon-Hydrogen also give better metallurgical effects on thicker stainless steel.

Nitrogen

Plasma cutting with pure Nitrogen is a strictly “thermal” process usually used on non-ferrous materials. On carbon steel using nitrogen as the plasma gas more dross is prevalent and nitriding or hardening of the cut edge is common.

Plasma cutting with pure Nitrogen is a strictly “thermal” process usually used on non-ferrous materials. On carbon steel using nitrogen as the plasma gas more dross is prevalent and nitriding or hardening of the cut edge is common.

Oxygen

Plasma cutting with oxygen is used to obtain faster cutting speeds at lower power levels on carbon steel and give you improved metallurgical effects on cut edge compared to Nitrogen or pure Air.

Plasma cutting with oxygen is used to obtain faster cutting speeds at lower power levels on carbon steel and give you improved metallurgical effects on cut edge compared to Nitrogen or pure Air.

Audit Services/Leak Detection

Our applications engineers can work with your plant personnel to analyse and understand your entire process. Based on that analysis and your needs, they can recommend process improvement solutions that can help you enhance product quality and consistency, plus optimise gas use. Air Products' services include leak-checking, furnace profiling, analytical calibration, gas analysis process troubleshooting and overall process review.

Our applications engineers can work with your plant personnel to analyse and understand your entire process. Based on that analysis and your needs, they can recommend process improvement solutions that can help you enhance product quality and consistency, plus optimise gas use. Air Products' services include leak-checking, furnace profiling, analytical calibration, gas analysis process troubleshooting and overall process review.

Training

Training can include gas atmosphere safety, properties of gases, metal treating applications, NFPA 86, piping and flow control panel requirements and troubleshooting for atmosphere problems. This information can help keep your furnace operations safe and help prevent accidents.

Training can include gas atmosphere safety, properties of gases, metal treating applications, NFPA 86, piping and flow control panel requirements and troubleshooting for atmosphere problems. This information can help keep your furnace operations safe and help prevent accidents.

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